An ultrasonic sensor is used to measure the traffic flow or detect cars waiting for a traffic signal. The ultrasonic sensor can detect only solid objects, which adequately reflect the ultrasonic wave. Therefore, the ultrasonic sensor is not suitable for detection of a person. Furthermore, the measurement area of the ultrasonic sensor is narrow. Therefore, it is difficult to detect the existence or the number of persons waiting for a red signal adjacent to the marked crosswalk, or the flow quantity of pedestrians going across by the blue signal on the marked crosswalk.
Accordingly, a method for monitoring pedestrians using an input image through TV camera is developed. As a monitoring method using a video, the following three methods are mainly considered. As a frame differential method, the difference between two images inputted at two different points in time is calculated to detect the moving area as the pedestrian. As a background image subtraction method, the difference between the input image and the background image previously prepared is calculated to detect the area whose brightness is different from the background as the pedestrian. As an optical flow method, motion of each pixel in the image is calculated to detect the area of the pedestrian, in which each pixel is moving straight at constant speed along a predetermined direction.
However, the above-mentioned three methods respectively include the following defects. The frame differential method is not applied to a person standing still such as a pedestrian waiting for the traffic signal. In the background image subtraction method, in the case of an outdoor monitor area, an update of the background image according to changes in sunlight is difficult. Furthermore, a shadow area of the pedestrian and a brightness change area on the road caused by a car headlight are included in the detection area. Therefore, these areas must be excluded. In the optical flow method, expensive calculation cost is required for extraction of the optical flow from the image. Furthermore, the person standing still is not detected by the optical flow method.